1. Field of the Invention
This invention relates to antennas and, more particularly, to a high-pass matching network for large, tapered dipole antennas.
2. Description of the Related Art
The following descriptions and examples are given as background only.
Large tapered dipole antennas (sometimes referred to as “biconical antennas”) have been used to generate intense electromagnetic fields, especially in the frequency range of about 20-300 MHz. While the radiated electromagnetic field of the antenna is useful, antenna designers have found that the near fields, especially the near electric fields (E-fields), may be exploited to produce even greater intensities than can be achieved with purely radiating fields.
In some cases, the near field may contain two subfield regions referred to as the reactive near field and the radiating near field. The radiating near field, also referred to as the Fresnel region, is the portion of the antenna field that lies between the reactive near field and the far field region, wherein radiation fields predominate and the angular field distribution is dependent upon the distance from the antenna. However, this region may not exist, if a maximum dimension of the antenna is small compared to the wavelength of the radiated electromagnetic energy.
Under most conditions, the near field does not extend very far from the dipole antenna. For an electrically-small dipole antenna, the near field consists almost entirely of the reactive near field, which extends only to a radial distance of about R=λ/2π, where λ is the wavelength of the radiated electromagnetic energy. When the antenna is not electrically-small, the near field consists of the reactive and radiating near field regions. In general, the radiating near field region may extend the near field to a radial distance, which is roughly half the physical length of the dipole. It, therefore, becomes clear that the radiating near field region may be extended by increasing the physical length of the antenna. Extending the radiating near field region is particularly useful in field generation for EMS testing, as it is often difficult to achieve requisite field intensities if near field components are not exploited.
In some cases, the near field intensity of a dipole antenna can be maximized by making the antenna length as long as physically possible (i.e., before mechanical interference prevents greater length) and as long as electrically possible (i.e., before antenna length causes the pattern to degrade to an unacceptable point). However, making the antenna as long as physically possible does not prevent the antenna from being electrically-small at the low end of its operating frequency range. Even though physically large, an antenna which is electrically-small at the low end of the operating frequency range will exhibit reduced performance within this range. In some cases, the antenna may be unable to generate acceptable field intensities within its low frequency range.
A need exists for a means to improve antenna performance at lower frequencies without reducing the performance of the antenna at higher frequencies. Such means would enable the antenna to maintain adequate performance over the entire operating frequency range.